This application claims the priority of German Application No. 100 11 419.9, filed Mar. 9, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an exhaust-gas turbocharger for an internal combustion engine.
A known exhaust-gas turbocharger is described in the German Patent document DE 36 28 687 A1. The exhaust-gas turbocharger comprises an exhaust-gas turbine which is arranged in the exhaust tract and is driven by the exhaust-gas back pressure of the internal combustion engine. Also, in the intake tract, a compressor, which is operated by the exhaust-gas turbine via a shaft, compresses fresh intake air to an increased boost pressure. The shaft of the exhaust-gas turbocharger is supported in the compressor casing via two rolling bearings. In order to achieve vibration-damped mounting, in each case at least one rolling bearing is supported in the compressor casing, with a gap-like damping space being included as a hydraulic cushion and/or with a radially acting spring body being included. This type of mounting constitutes a spring damper system which makes it possible to avoid the situation where the critical rotational speed of the shaft may be set in the range near the maximum rotational speed of the exhaust-gas turbocharger.
Rotor mountings of this type have, in principle, incorporated a potential for improvement in terms of turbocharger efficiency in connection with the friction occurring at the bearing point between the shaft and the bearing receptacle. Particularly in the range of low turbocharger rotational speeds, a considerable percentage of the turbine power is lost in the form of bearing power loss, so that, in the part-load range of the internal combustion engine, a merely reduced boost pressure may be built up, this manifesting itself in a delayed response behavior of the internal combustion engine during acceleration.
Another problem is that the bearings have to be supplied with lubricating oil, which presupposes an oil pump and an oil supply line to the bearings. The supply of oil normally takes place via the oil circuit of the internal combustion engine. For reasons of construction, the situation cannot be ruled out where leakages occur both on the turbine side and on the compressor side in the exhaust-gas turbocharger, with lubricating oil escaping into the exhaust-gas side or into the air side of the engine via these leakages. In addition to the air and the exhaust gas being polluted with lubricating oil, there is also the fear that various components of the internal combustion engine, for example a charge-air and exhaust-gas recirculation cooler or a soot filter, may be contaminated with oil.
The undesirable infiltration of the lubricating oil into air and exhaust gas or the contamination of various components with oil necessarily represent a loss of lubricating oil which has to be regularly compensated. Moreover, the high temperatures on the exhaust-gas side in the region of the turbine damage the oil, and the useful life of the oil is reduced.
The problem on which the invention is based is to improve the efficiency of an exhaust-gas turbocharger for an internal combustion engine. The ease with which the internal combustion engine is maintained and the useful life of the exhaust-gas turbocharger are also expediently to be improved.
This problem is solved, according to the invention, by an exhaust-gas turbocharger for an internal combustion engine, comprising an exhaust-gas turbine in the exhaust tract, and a compressor in the intake tract. The exhaust-gas turbine and the compressor are connected via a shaft which is supported in a casing of the exhaust-gas turbocharger via at least one bearing. The bearing has a non-contact design, such that, when the exhaust-gas turbocharger is in operation, the shaft is held at a distance from, and so as to be virtually free of friction with, a bearing receptacle fixed to the casing.
According to the innovation, the bearing of the rotor shaft of the exhaust-gas turbocharger in the casing has a non-contact design, in that, at least when the exhaust-gas turbocharger is in operation, the shaft is held at a distance, with a bearing gap, from the bearing receptacle fixed to the casing. A virtually friction-free mounting of the shaft in this case is thereby possible, with the result that the efficiency of the turbocharger, particularly at low rotational speeds, is appreciably improved, since virtually no bearing losses occur any longer. The response behavior of the internal combustion engine is improved, because, even in the low rotational-speed range of the internal combustion engine and with a correspondingly low exhaust-gas back pressure, a notable turbine power can be generated, which is transmitted via the shaft to the compressor. Thus, even in the lower rotational-speed range, this brings about an increase in the boost pressure and therefore, concomitantly, an increase in the power of the internal combustion engine.
Moreover, the result of the friction-free mounting is that the use of lubricating oil may be dispensed with. As a consequence of which, on the one hand, the design of the exhaust-gas turbocharger is appreciably simplified, because devices for supplying oil to the turbocharger are no longer required and, on the other hand, the problem of undesirable contamination of the intake air, the exhaust gas or various assemblies of the internal combustion engine with oil, is avoided. Furthermore, there is no fear that the quality of the engine oil will be impaired because the oil is heated to an undesirable extent, nor do any additional oil losses occur.
According to a first advantageous development, the bearing is designed as an air bearing, in which an air gap is formed between the shaft and the bearing receptacle fixed to the casing. Air, which can expediently be supplied via an air supply device, flows into the air gap, the air bearing advantageously being designed as an aerodynamic bearing, in which the supplied air flows through the air gap, with the result that a supporting air cushion is generated. As a result of the compressibility of the air, when the shaft rotates, an over pressure zone and an under pressure zone are formed, the pressure difference ensuring adjustment or centering of the shaft in the bearing receptacle.
Alternatively to an aerodynamic bearing, an aerostatic bearing may also be used, in which air is pressed into the bearing gap from outside. When the bearing is under load, a higher pressure is formed in the narrowed bearing gap than in the widened bearing gap; this pressure difference gives rise to the load-bearing capacity. Aerostatic bearings may expediently be used in the case of shafts in which a relatively high load is to be expected.
According to a second advantageous version, which may be used both alternatively to and in addition to the air bearing, the bearing is designed as a magnetic bearing and forms, in particular, part of an electric motor, in which the shaft of the exhaust-gas turbocharger or a component connected to and rotating with the shaft is identical to the rotor or armature of the electric motor. The design as an electric motor affords the advantage that, in addition to the non-contact and friction-free mounting of the shaft, an additional drive of the shaft is provided, which, particularly in ranges of low exhaust-gas back pressure, can be cut-in in order to achieve an increase in boost pressure. By contrast, in ranges of higher exhaust-gas back pressure, the electric motor can be operated as a generator.
In a preferred embodiment, an additional contact bearing, in particular a rolling bearing, is provided, which takes effect in operating states where the supporting force of the non-contact bearing is not sufficient. Particularly when an air bearing is used, the contact bearing affords the advantage that the dry or mixed friction that occurs when an aerodynamic air bearing is started up, and that leads to an increased starting torque and wear can be compensated or reduced by means of the contact bearing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.